This invention relates to stable, aqueous latexes and to methods for their preparation.
Aqueous dispersions of polymers, which are referred to in the art as latexes, are generally known to be useful, both alone and in various formulations, as coatings and impregnants. A wide variety of latexes of differing homopolymeric and copolymeric compositions (such as styrene-butadiene copolymers, acrylic homopolymers and copolymers, vinylidene chloride homopolymers and copolymers, etc.) have been developed having specific chemical and/or mechanical properties for particular end use applications. For example, aqueous interpolymer latexes resulting from the emulsion polymerization of monovinyl aromatic monomers, such as styrene; diolefins, such as butadiene, and monoethylenically unsaturated carboxylic acids, such as acrylic acid, are known to be particularly useful as film-forming binders for pigments in paper coating applications. See, for example U.S. Pat. Nos. 3,399,080 and 3,404,116. Such emulsion polymerizations optionally employ conventional seeding procedures for optimum control of polymerization and in order to obtain maximum product uniformity (e.g., narrow particle size distribution).
U.S. Pat. No. 4,151,143, issued to Blank et al., discloses a surfactant-free polymer emulsion coating composition and a method for preparing the same. Blank et al. identify that a main problem with emulsion polymers employed for coatings is the presence of surfactants which are employed to stabilize emulsions but which adversely affect the water and corrosion resistance of the resulting film as well as the adhesion of the coating especially to metal surfaces. The Blank et al. emulsion polymers are prepared in a two stage process. The process includes (1) a first stage wherein a conventional carboxyl group containing polymer is prepared by a conventional solution or bulk polymerization technique and thereafter water-dispersed or solubilized by partial or full neutralization thereof with an organic amine or base and application of high shear agitation and (2) a second stage wherein a mixture of polymerizable monomers and polymerization catalyst is added to the first stage emulsion at an elevated temperature to effect polymerization of the stage two monomers resulting in the formation of an emulsion coating composition. The coating composition is surfactant-free.
U.S. Pat. No. 4,179,417, issued to Sunada et al., discloses a composition for water based paints containing a water soluble resin and a water dispersible polymer. The water soluble resin contains 50-99.5 percent by weight of an alpha, beta-monoethylenically unsaturated acid alkyl ester to an alkenyl benzene, 0.5-20 percent by weight of an alpha, beta-monoethylenically unsaturated acid and 0-30 percent by weight of a hydroxyalkyl ester of an alpha, beta-monoethylenically unsaturated acid. These monomers are polymerized in the presence of an alkyd resin containing a polymerizable unsaturated group, epoxy esters containing a polymerizable group, drying oils, fatty acids of drying oils and diene polymers. The resulting polymers are water solubilized by the addition of ammonia or an amine. The water dispersible polymer contains hydroxy and/or carboxyl functional groups and contains an alpha, beta-monoethylenically unsaturated acid monomer and/or a hydroxy alkyl ester of such a monomer and other ethylenically unsaturated monomers. The compositions disclosed in U.S. Pat. No. 4,179,417 are employed in water based paints and can optionally contain a cross linking agent.
Canadian Pat. 814,528, issued June 3, 1969, discloses low molecular weight alkali soluble resin, resin solutions and methods of their preparation and purification. The resins are disclosed as being especially useful as emulsifiers, leveling agents, and film-formers. The number average molecular weight of the resins range from 700-5000 and the resins have acid numbers between 140-300. The resins are disclosed as emulsifiers in the preparation of emulsion polymers resulting in emulsion polymers which are stable and substantially free from coagulum. For this use, i.e., emulsifier in emulsion polymerization reactions, the resins must have a number average molecular weight between 1,000 and 2,000 and preferably between 1,000 and 1,500. Resins having a number average molecular weight greater than 2,000 may lead to unstable and coagulated emulsion polymers when used as the emulsifier in emulsion polymerizatio reactions.
Two stage latex polymers are known to exist in many morphological forms, which are determined by many factors including the relative hydrophilicity, miscibility and molecular weights of the first and second stage polymers. So-called "core-shell" latexes are formed when the second stage polymer forms a "shell" or coating around a discrete domain or "core" of the first stage polymer. Examples of such core-shell latexes are disclosed in U.S. Pat. No. 4,515,914 where an exemplary composition contains a first stage styrene/butadiene core polymer which is encapsulated by a shell of a second stage monovinyl polymer. "Inverted core-shell" latexes are also known. Lee and Ishikawa, "The Formation of `Inverted` Core-Shell Latexes, "J. Poly. Sci., 21, 147-154 (1983) shows that such "inverted" latexes are those where the second stage polymer becomes the core domain and is encapsulated by the shell first stage polymer. These inverted latex compositions can be formed when the first stage polymer is more hydrophilic than the second stage polymer. Lee and Ishikawa studied the formation of the "inverted" core-shell morphology using two polymer pairs; a dsoft polymer pair [ethyl acrylate/methacrylic acid (EA/MAA) (90/10)]/[styrene/butadiene (S/B)(60/40)] and a hard polymer pair [EA/S/MAA (50/40/10)]/[S (100)]. The ratio of monomers in the polymers is in parts by weight. Soft polymers have a relatively low glass transition temperature (Tg), generally below about 20.degree. C., while hard polymers have a relatively high Tg, generally above about 20.degree. C. It was found that in the case of the soft polymer pair systems, the formation of inverted core-shell morphology was equally complete, regardless of the molecular weight of the hydrophilic polymer molecules, whereas in the case of the hard polymer pair systems, the efficiency of inversion depended upon the molecular weights of the hydrophilic and hydrophobic polymers. The study suggests that the formation of inverted core-shell latexes depends not only on the hydrophilicity, interfacial tension, and molecular weight of the hydrophilic polymer molecules, but also on the extent of phase separation between two polymers.
Lee and Ishikawa also noted the alkali swellability of the first stage polymer in these inverted emulsion systems.
Muroi, et al. "Morphology of Core-Shell Latex Particles," J. Poly. Sci., 22, 1365-1372 (1984) evaluated latex particles formed when an ethyl acrylate-methacrylic acid or methyl acrylate (MA)-MAA mixture was polymerized in the presence of poly (MA-MAA) or poly (EA-MAA) seeds. They found that the shell was composed of the more hydrophilic poly (MA-MAA) molecules which were relatively high in MAA content and the core was composed of both poly (MA-MAA) and poly (EA-MAA) molecules, with the copolymeric particles being relatively uniform from surface to center in distribution of all components except MAA. The monomer content of MAA increased in the direction of the shell surface.
In particular, Muroi, et al. studied five compositions, including one where the first stage feed was MA/MAA (90/10) and the second stage feed was EA/MAA (90/10). The authors found that as the pH of the resulting latex was increased through the additon of NaOH, the optical density decreased, indicating complete dissolution of all the latex particles.
A commonly assigned, copending patent application Ser. No. 899,281 filed Aug. 22, 1986 is directed to emulsions formed by adding a preformed support resin during emulsion copolymerization of monomers adapted to form low molecular weight anionic polymers, such as the ammonia salt of styrene/acrylic acid polymer. Films produced from such emulsions are sometimes unduly brittle and subject to alkali attack, which is particularly a problem when the emulsion is designed for use as a polish or varnish. In addition the anionic polymers must be present in substantial amounts to stabilize the support resin and accordingly, act as polymeric surfactants.
In view of the above it is desirable to provide a stable latex emulsion capable of employing a relatively broad spectrum of hard and soft monomers and basic functional, as well as acid functional monomers.